1. Field of the Invention
This invention relates to the field of piezoelectricity and, more particularly, to an improved construction for and improved method of manufacturing piezoelectric crystal assemblies of the general class used for frequency controlling purposes in radio equipment, television equipment and diverse other types of electronic equipment.
2. Description of Prior Art
Generally speaking, piezoelectric crystal assemblies include a crystal element cut or formed from quartz or other piezoelectrically active material into a relatively thin circular disc or rectangular plate having opposed major faces, electrically conductive electrode structures respectively mounted upon or otherwise held in engagement or close proximity with the major faces of the crystal element, some form of protective housing or cover, and some means for supporting the crystal element (and the electrode structures, if they are not secured to the crystal element) within the protective housing or cover in a manner that will not interfere with or unduly damp the desired resonantly vibratory motion of the crystal element.
Until about the 1930's, piezoelectricity was largely treated as a laboratory phenomenon, and those crystal assemblies that existed were relatively crude and typically characterized by the whims of the particular experimenters who built them or the materials that they may have had readily available for such purpose.
As the commercial, military and marine useage of radio communications developed and expanded, however, the advantages of employing the resonance properties of piezoelectric crystals for more stably controlling the frequency of oscillation of the electronic oscillators being incorporated into the exciter stage of radio transmitters and the like spawned a need for the availability in commercial quantities of more reliable and less individualistic types of piezoelectric crystal assemblies. At that time, quartz crystal elements of "cuts" (from a natural mother crystal) adapted to vibrate in longitudinal or other modes now considered non-optimum for most purposes were popular and were fabricated in the form of rectangular plates typically having major dimensions approaching one inch or more. Accordingly, the electrode structures were then usually implemented as electrically conductive metal plates pressed into contact with the major surfaces of the crystal element by some form of metallic spring means through which electrical connections with the electrodes could also be established. One such early construction employed a base and an integral housing frame thereon of electrically insulative material such as bakelite, a rectangular aperture through the frame for receiving the crystal element and a pair of shiftable metal electrode plates respectively contacting the major faces of the crystal element therebetween, a metal side wall mounted on each side of the frame, metal compression springs between at least one of the side walls and the adjacent shiftable electrode plate, and a pair of terminal pins carried by the base and suitably coupled with the electrodes via the springs or otherwise.
Further development of piezoelectric crystal assemblies then evolved along lines including the employment of thin electrically conductive metal coatings deposited in adhering relationship upon the major faces of the crystal element to take the place of separate electrode structures, the supporting of the crystal element by means of wires directly connected to the deposited electrode coatings, and increased usage of so-called "thickness shear" vibrating quartz elements (such as those commonly referred to as "AT" and "BT" cuts) in order to take advantage of the higher resonance frequency characteristics and the greater frequency/temperature stability characteristics of those types of quartz crystal elements. The trend toward almost universal employment of such thickness shear vibrating crystal elements was attended by somewhat differing considerations applicable to the packaging thereof into crystal assemblies, than had been the case for the earlier crystals that vibrated in longitudinal, flexure or other modes. First, it was found that thickness shear mode type quartz crystal elements would operate efficiently when fabricated with much smaller dimensions than had been required for the types of crystal elements earlier utilized. Secondly, it was found that the thickness shear mode type crystal elements could most advantageously be fabricated in the form of circular discs. Thirdly, since the nodal areas of a thickness shear mode vibrating type crystal element are different than for the types of crystal elements earlier utilized (the piezoelectric activity of primary utility in a thickness shear mode type crystal element being at the central areas of the major faces thereof), and since it is necessary for greatest piezoelectric efficiency for a crystal element to be supported at areas thereof which are either nodal or relatively inactive from the vibratory standpoint (adjacent the edge of a disc type thickness shear mode element and, if of a relatively large diameter, preferably also adjacent the central thickness plane thereof), techniques for supporting the thickness shear crystal elements needed to be altered from those previously employed for the earlier types of crystal elements.
Accordingly, essentially from the beginning of widespread usage of disc type thickness shear mode crystal elements, the electrode structures employed therewith have typically been of the deposited metallic coating type. The approach initially employed in the supporting of such crystals commonly utilized the wires that were connected to the electrode coatings for electrical purposes for also providing physical support for the crystal element itself, such wires in turn typically being soldered to terminal pins in a non-conductive base, with a hollow cap of metal being mounted on the base to provide a protective enclosure for the crystal element. Since the early practice of supporting thickness shear mode type crystals by wires directly connected with the deposited electrode coatings thereon caused some damping of the vibratory action of the crystal element by virtue of the physical connections made therewith at the areas of primary piezoelectric activity of such crystals, the electrode coatings were soon thereafter provided with intergrally deposited tab portions extending radially outwardly from the main circular central portion of each electrode coating toward the edge of the crystal disc, in order that both the required electrical connections to the electrodes and physical support for the crystal element could be provided adjacent the more nodal edge area of the crystal element.
The housings in which such thickness shear mode crystal elements were packaged then evolved along essentially two lines. One approach was to continue the use of an electrically insulative base carrying a pair of terminal pins to which a pair of connecting and supporting wires would be respectively fastened, with such wires then being soldered or otherwise attached to the extension tabs of the electrode coatings adjacent the edge of the crystal element, and with a cap of metal or the like mounted on the base and sometimes hermetically sealed to the latter for protectively enclosing the crystal element. The other approach, which ultimately became the more dominant, involved various constructions essentially employing a central annular sleeve of electrically insulative material within which the crystal element was received and supported by means of metallic caps mounted on both sides of the sleeve and having some means, such as separate springs or resilient fingers on the caps, for oppositely engaging the crystal element adjacent the edge of its major faces, including the marginal portion thereof to which the radial tab portions of the electrode coatings extended for effecting electrical connections therewith. In order to provide moisture-proofing in the last mentioned type of assembly, it then became accepted practice to encapsulate the crystal element housing with a layer of hardened epoxy material from which only the electrical connecting leads secured to the end caps protruded.
The McGrew U.S. Pat. No. 3,622,816, issued to a predecessor of the assignee of this patent, discloses the last mentioned type of crystal assembly, which it is believed can be fairly regarded as representing essentially the preferred state of the art construction for crystal assemblies existing and prevalent as a matter of commercial practice prior to the present invention.
Despite the fact that the piezoelectric crystal assembly construction provided by the McGrew patent has proved to be highly satisfactory from the operational standpoint, it will be observed from the disclosure of that patent that the construction contemplated thereby requires fabrication and proper assembly of a number of separate structural parts. Accordingly, especially in view of the fragile nature of the quartz crystal elements employed and the miniaturized nature of all of the structural components involved, it will be appreciated that the fabrication and assembly of such devices has remained relatively time consuming and relatively costly, with many operations being involved which are of nature heretofore regarded as requiring individualized attention by skilled personnel for each unit to be produced. It will also be appreciated that the number and nature of the structural parts and the manner in which they must be fabricated and assembled for each unit produced with such constructions tends to render the maintenance of proper quality control and uniformity a significant task, particularly in view of the large quantity of piezoelectric assemblies that are now required to satisfy the demands for such assemblies for use in television receivers and many other types of equipment.
These, then, are the primary practical problems characterising the state of the prior art to whose solution the present invention is directed, while at the same time also providing improvements from the operational and reliability standpoints.
The other known prior art disclosures which may be of some incidental interest in connection with certain details of the construction involved in the present invention are U.S. Pat. Nos. 3,656,217 and 3,849,681 showing the use of wires or rods having forked or bifurcated ends for making both electrical and physical supporting connections with a crystal element; U.S. Pat. Nos. 3,747,176, 4,017,752 and 4,103,264 showing a technique for providing a pocket of air or gas surrounding an encapsulated crystal element in which the crystal element is initially covered with a heat dissipatable material, then encapsulated, and then heated to cause the material to dissipate or be reduced to gaseous form in a pocket adjacent the crystal element within the encapsulation body; and published Japanese Patent Application 124,981 of Ishibashi et al, filed Oct. 16, 1975, Publication No. 48489/77, apparently relating to a crystal assembly employing a rectangular "porcelain" crystal element of the longitudinal or flexure mode type having deposited electrodes and supported at end portions of one face thereof upon a larger rectangular base of insulative material provided with conductive pads for contacting the electrodes, in which an air containing bag is formed around the entire element and base assembly by heat welding the edges of a pair of thermoplastic sheets between which it is sandwiched, with such bag then being covered with an outer hardened encapsulation layer.
Insofar as is known, however, neither prior practices nor prior disclosures of others have either taught or suggested the improved construction or method of manufacture provided by this invention.